As a working process in which metallic materials such as iron and steel materials and stainless steels are plastically deformed, mainly, there are hot forging in which a steel material is molded while being heated and cold forging in which a steel material is molded using a mold at room temperature.
In recent years, efforts have been being made to decrease weights of automobile bodies in order to reduce amount of CO2 emissions from the automobiles from the viewpoint of global environmental protection, and a use of a high-strength steel plate having a strength of 440 MPa or more is proceeded. In addition, in automobile companies and parts makers, parts which were conventionally produced through hot forging are produced through cold press forging so as to simplify production steps. Simplification of steps saves energy and decreases costs in the production process; and thereby, efficiency of the process is improved. Particularly, from the viewpoint of improving the efficiency of the production process, a production method in which a plate material is subjected to cold press forging without conducting hot forging, that is, plate press forging is applied to a process of producing parts which were conventionally formed by subjecting a material such as a steel bar and the like to hot forging and cutting work so as to secure part accuracy.
However, in the case where a 440 MPa or higher-class plate material is subjected to cold plate press forging, a problem that material cracks occur is notably caused compared to hot forging. In addition, uneven formability due to rolling-induced anisotropy in the plate surface is observed. The uneven formability does not occur easily in an axially symmetric material such as a steel bar. There are a lot of problems that need to be solved such as the occurrence of cracking in a specific direction and unevenness in shape after working. At the moment, it is necessary to change a design to a shape in which cracking does not occur, and it is also necessary to carry out a step in which uneven portions occurred after drawing, so-called ear portions, are cut off. Therefore, there is a demand for a material having better workability and uniform characteristics.
As described above, in the process of producing parts, it is necessary to improve workability which is required for a material in order to greatly simplify the process steps compared to the related art. Particularly, in order to change the material from a steel bar to a steel plate, there has been a demand for an improvement of anisotropy between a rolling direction and a direction perpendicular thereto.
Particularly, unlike pressing of a steel plate having a thickness of approximately 1 mm in the related art, cold plate press forging is performed on a hot-rolled steel plate having a thickness of approximately 2 mm to 25 mm as a material for parts such as engines, transmissions, and the like, and the hot-rolled steel plate is thicker than a steel plate used for body parts in the related art. Therefore, ultimate deformability that is required during working is an important characteristic.
As a high-strength hot-rolled steel plate that is excellent in ultimate deformability and shape fixability, a hot-rolled steel plate is proposed which is obtained by controlling texture and anisotropy in ductility (for example, refer to Patent Document 1). However, Patent Document 1 does not specifically disclose cold plate press forging.
In addition, cold forging attains extremely high productivity and dimensional accuracy. In addition, a worked product worked through cold forging has advantages such as improved abrasion properties, enhanced strength due to cold work hardening, and the like. However, in cold forging, a metallic material is pressed while the metallic material is brought into contact with a mold or the like at a high surface pressure. As a result, temperature at the contact portion between the metallic material and the mold becomes a relatively high temperature (approximately 300° C. or higher) due to friction between the metallic material and the mold during pressing. Therefore, in the case where lubricity between the metallic material and the mold is not sufficient, such as the case where a metallic material that is not surface-treated or the like is subjected to cold forging, there are cases in which seizure or galling occurs between the metallic material (material) and the mold. Seizure or galling causes local breakage or abrupt abrasion of the mold; and thereby, not only there are cases in which the service life of the mold is greatly shortened, but also there are cases in which working becomes impossible.
In order to prevent seizure or galling, generally, a metallic material to be subjected to cold forging is subjected to a surface treatment for applying lubricity to a surface of the metallic material (hereinafter often referred to as “lubrication treatment”). As the lubrication treatment, a phosphate treatment (bonderizing treatment) has been known in the related art in which a phosphate film composed of a phosphate compound (zinc phosphate, manganese phosphate, calcium phosphate, iron phosphate, or the like) is formed on a surface of a metallic material.
Performance of the phosphate treatment to prevent seizure and galling is relatively strong. However, as described above, due to the recent environmental measures, cold forging is more commonly carried out than workings that involve large shape deformation, such as hot forging accompanied by large energy consumption and cutting work that causes a large amount of material loss, and there is a demand for stricter plastic working in cold forging. From the above-described viewpoint, a composite film has been widely used which further includes a layer composed of a metallic soap (for example, sodium stearate or the like) laminated on the phosphate film. The composite film has an excellent performance to prevent seizure and galling even under strict abrasion conditions due to pressing with a high surface pressure during cold forging.
According to the lubrication treatment to form the composite film, the metallic soap reacts with the phosphate film; and thereby, favorable lubricity is exhibited. However, the lubrication treatment requires a lot of cumbersome treatment steps such as a cleaning step, a reaction step in which the metallic soap and the phosphate film are reacted with each other, and the like. In the reaction step, it is also necessary to control a treatment fluid, a temperature during the reaction, and the like. In addition, since the lubrication treatment is a batch treatment, there is a problem in that the productivity degrades. In addition, the lubrication treatment to form the composite film has problems such as a treatment of a waste liquid generated during the treatment or the like, and the lubrication treatment is not preferred from the viewpoint of environmental protection.
Therefore, in recent years, a variety of lubrication treatment processes have been proposed for replacing the lubrication treatment to form the composite film.
For example, Patent Document 2 proposes a lubricant composition or the like in which a water-soluble polymer or a water-based emulsion thereof is included as a base material, and a solid lubricant and an agent for forming a chemical conversion coating film are further included. However, with regard to the lubricant composition or the like of Patent Document 2, lubricity and performance to prevent seizure and galling that are comparable to those of the above-described composite film cannot be obtained.
In addition, for example, Patent Document 3 proposes a water-based lubricant for cold plastic working of metal. The water-based lubricant is composed of (A) a water-soluble inorganic salt, (B) a solid lubricant, (C) at least one oil component selected from a mineral oil, an animal or plant fat, and a synthetic oil, (D) a surfactant, and (E) water, and the solid lubricant and the oil component are uniformly dispersed and emulsified respectively. However, since the oil component is emulsified, the lubricant obtained by the above-described technique is unstable for industrial use, and favorable lubricity is not stably exhibited.
In contrast to the above-described matters, for example, Patent Document 4 proposes a metallic material for plastic working which includes a concentration-gradient type two-layer lubricant film composed of a base layer and a lubricant layer. Patent Document 4 describes that a film having favorable lubricity can be generated through a simple treatment.
However, in the technique of Patent Document 4, adhesion between the film and a metal which is a base material is insufficient; and thereby, the film easily separates from the metal during working, particularly during strong working. Since a mold and the metal come into contact with each other at portions where the film separates, there is a problem in that seizure easily occurs at the separation portions.